Alzheimer's disease (AD) is one of the most rapidly growing disorders in developed countries. Four million Americans have been diagnosed with this disorder, and as the population grows older and life expectancy increases, the prevalence is expected to double in the next 20 years. Yet at the present time there are no truly effective therapeutics to prevent or even retard the development of AD. The risk factor or susceptibility gene proven to play a major role in AD pathogenesis is the presence of apolipoprotein (apo) E4. ApoE4 is clearly established to increase the occurrence and lower the age of onset of the sporadic and familial forms of late-onset AD. The number of AD patients carrying at least one apoE4 allele ranges from 50-80% in different population studies (the apoE4 allele occurs in approximately 15% of the general population). Studies of the structure and function of apoE have revealed that apoE4 possesses a unique conformation characterized by domain interaction, i.e., arginine 61 in the amino terminus and glutamic acid 255 in the carboxyl terminus interact. ApoE3, the most common apoE isoform, is incapable of domain interaction and has a more open conformation. Many functional properties of apoE4 that distinguish it from apoE3 appear to be modulated by domain interaction. Thus, the hypothesis driving this proposal is that if we could prevent apoE4 from assuming its detrimental structural conformation, we might block the apoE4-related neuropathology. We will use fluorescence resonance energy transfer (FRET) (Specific Aim 1) to identify small molecules capable of preventing domain interaction in apoE4. Screening of chemical libraries in combination with the FRET assay will identify compounds that convert apoE4 to an "apoE3 1ike" molecule structurally and functionally. In addition we will use a fluorescence-release assay to identify small molecules capable of preventing apoE4 destabilization of phospholipid-rich membranes (Specific Aim 2). ApoE4 destabilization of intracellular membranes represents one mechanism whereby apoE4 could be related to neuropathology. Use of these assays in chemical library screening will increase our chances of identifying small molecules ("hits") that act through different mechanisms to modulate apoE4 conformation and related neuropathology. Ultimately, it will be our goal to take "hits" to lead compounds that may give rise to a drug that prevents or retards apoE4-related neuropathology.